+/*
+ * Within this solver, I'm going to transform all y-coordinates by
+ * inverting the significance of the block number and the position
+ * within the block. That is, we will start with the top row of
+ * each block in order, then the second row of each block in order,
+ * etc.
+ *
+ * This transformation has the enormous advantage that it means
+ * every row, column _and_ block is described by an arithmetic
+ * progression of coordinates within the cubic array, so that I can
+ * use the same very simple function to do blockwise, row-wise and
+ * column-wise elimination.
+ */
+#define YTRANS(y) (((y)%c)*r+(y)/c)
+#define YUNTRANS(y) (((y)%r)*c+(y)/r)
+